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Arachin 8a teaches the rules of what happens when a woman has a flow of blood and is unsure if she is in her days of niddah (days 1 - 7) or days of zivah (days 11 - 18).

For simplicity, let’s focus on the case where she has a flow for five consecutive days, and follow Rashi’s interpretation. The ‘worst-case scenario’ is that the first two days belong to the previous zivah cycle (days 17 - 18), and the next three days are part of the subsequent niddah period. In which case, she needs to wait fifteen clean days (days 4 - 18), after which any flow will render her a niddah and not a zavah.

She need not wait any longer than that, because if the flow was for three or more consecutive days during her zivah period, she would automatically re-enter her niddah days after seven clean days, and only after seven clean days.

Now, Rambam in Hilchot Issurei Bi’ah 6:6 has a completely different view of the niddah/zivah cycle. Essentially he holds that they simply repeat in the 7-11-7-11 pattern more or less indefinitely, whether or not she has any flow during the eleven days, and, if she does have a zivah flow, whether or not she succeeds in counting seven clean days. (And see e.g. Beit Yosef Yoreh De’ah 183 who explicitly understands Rambam this way.)

So, how does Rambam understand Arachin 8a? She had a flow for five days. How is waiting fifteen clean days sufficient to get her back on track, and knowing that a subsequent flow certainly renders her a niddah? Maybe those five days are days 14 - 18? Then, according to Rambam, day 19 is actually day 1 of the next niddah cycle, even without seven clean days, so she waits fifteen clean days and has a flow again on day 16, back in the middle of her next zivah period?

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The 'חוות דעת סי' קפ"ג ביאורים ס"ק ב answers your question on the Rambam.

A very short summary of his answer would be:

According to the Rambam, Arachin 8a is dealing with trying to determine when a vest can be established.

1 - The Rambam's rule of 7/11 always repeating itself, is only if she does not have a vest. Should she have a vest, niddah days followed by ziva days start from the vest, and not from her previous cycle.

2 - She cannot establish a vest during the ziva days.

3 - If she doesn't know where in her cycle she is, she will have to wait the amount of days mention in Arachin 8a, to be able to create a vest and a new 7/11 cycle.

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